1. Field of the Invention
The invention relates to mud driven motors used in the drilling of oil wells. More particularly, the invention relates to a downhole drilling motor which has a wholly non-elastomeric stator and rotor.
2. Description of the Related Art
The idea of downhole motors for driving an oil well drill bit is more than one hundred years old. Modern downhole motors are powered by circulating drilling fluid (mud) which also acts as a lubricant and coolant for the drill bit. Prior art FIG. 1 shows a state-of-the-art downhole motor assembly. The assembly 10 generally includes a rotatable drill bit 12, a bearing/stabilizer section 14, a transmission section 16 which may include an adjustable bent housing (for directional drilling), a motor power section 18, and a motor dump valve 20. The bent housing 16 and the dump valve 20 are not essential parts of the downhole motor. As mentioned above, the bent housing is only used in directional drilling. The dump valve is used to allow drilling fluid to enter the motor as it is lowered into the borehole and to allow drilling fluid to exit the motor when it is pulled out of the borehole. The dump valve also shuts the motor off when the drilling fluid flow rate drops below a threshold. During operation, drilling fluid pumped through the drill string (not shown) from the drilling rig at the earth's surface enters through the dump valve 20, passes through the motor power section 18 and exits the assembly 10 through the drill bit 12.
Prior art FIGS. 2 and 3 show details of the power section 18 of the downhole motor. The power section 18 generally includes a housing 22 which houses a motor stator 24 within which a motor rotor 26 is rotationally mounted. The power section 18 converts hydraulic energy into rotational energy by reverse application of the Moineau pump principle. The stator 24 has a plurality of helical lobes, 24a-24e, which define a corresponding number of helical cavities, 24a'-24e'. The rotor 26 has a plurality of lobes, 26a-26d, which number one fewer than the stator lobes and which define a corresponding plurality of helical cavities 26a'-26d'. Generally, the greater the number of lobes on the rotor and stator, the greater the torque generated by the motor. Fewer lobes will generate less torque but will permit the rotor to rotate at a higher speed. The torque output by the motor is also dependent on the number of "stages" of the motor, a "stage" being one complete spiral of the stator helix.
In state-of-the-art motors, the stator 24 is made of an elastomeric lining which is molded into the bore of the housing 22. The rotor and stator are usually dimensioned to form a positive interference fit under expected operating conditions, as shown at 25 in prior art FIG. 4. The rotor 26 and stator 24 thereby form continuous seals along their matching contact points which define a number of progressive helical cavities. When drilling fluid (mud) is forced through these cavities, it causes the rotor 26 to rotate relative to the stator 24. The interference fit 25 is defined by the difference between the mean diameter of the rotor 26 and the minor diameter of the stator 24 (diameter of a circle inscribed by the stator lobe peaks). Motors which have a positive interference fit of more than about 0.559 millimeters (0.022 inches) are very strong (capable of producing large pressure drops) under downhole conditions. However, a large positive interference fit will provoke an early motor failure. This failure mode is referred to as "chunking".
Interference fit is believed to be critical to the performance and overall life of the motor. In practice, the magnitude of the interference fit (at the time of assembly) is dictated by the expected temperature of the drilling fluid and downhole pressure. High temperatures will cause the elastomeric stator of a motor with negative or zero interference fit to expand and form a positive interference fit. For use at lower temperatures, it is necessary to assemble the motor with a positive interference fit. As mentioned above, a motor with excessive interference fit will fail early. On the other hand, a motor with insufficient interference fit will be a weak motor which stalls at relatively low differential pressure. A motor stalls when the torque required to turn the drill bit is greater than the torque produced by the motor. When this happens, mud is pumped across the seal faces between the rotor and the stator. The lobe profile of the stator must then deform for the fluid to pass across the seal faces. This results in very high fluid velocity across the deformed stator lobes.
In addition to temperature, certain types of drilling fluids may have an adverse effect on the operation of the drilling motor. For example, certain types of oil-based drilling fluid and drilling fluid additives can cause elastomeric stators to swell and become weak. Therefore, the composition of the drilling fluid must also be considered when choosing a motor with the appropriate amount of interference fit.
Those skilled in the art will appreciate that the elastomeric stator of drilling motors is a vulnerable component and is responsible for many motor failures. However, it is generally accepted that either or both the rotor and stator must be made compliant in order to form a hydraulic seal. As mentioned above, it is generally believed that without sufficient positive interference (hydraulic seal) between the rotor and stator, the motor will be weak (generate low torque) and will easily stall.